What is the initial work‑up for a patient suspected of having Cushing's disease?

Initial Work-Up for Suspected Cushing's Disease

Begin by obtaining a detailed medication history to exclude exogenous glucocorticoid exposure from oral, inhaled, topical, or injected sources before ordering any biochemical tests. 1, 2

First-Line Screening Tests

You must perform at least two different screening tests to confirm the diagnosis before proceeding to imaging or localization studies. 3, 2 Choose from the following based on patient-specific factors:

24-Hour Urinary Free Cortisol (UFC)

• Collect at least two to three separate 24-hour urine samples on different days to account for day-to-day variability. 1, 2, 4
• Verify adequate collection by checking urinary creatinine excretion. 4
• **Avoid UFC if creatinine clearance is <60 mL/min** or if the patient has marked polyuria (>5 L/24 hours), as results become unreliable. 2, 4
• UFC values >3-fold the upper limit of normal strongly suggest true Cushing's syndrome rather than pseudo-Cushing states. 2

Late-Night Salivary Cortisol (LNSC)

• Collect at least two samples between 11 PM and midnight on separate nights. 1, 2
• This test is preferred for shift workers, suspected cyclic Cushing's syndrome, and outpatient convenience because it captures loss of the normal circadian rhythm. 2
• LNSC is particularly useful when UFC collection is impractical or unreliable. 1, 2

1-mg Overnight Dexamethasone Suppression Test (DST)

• Administer 1 mg dexamethasone at 11 PM and measure serum cortisol at 8 AM the next morning. 1, 2
• A cortisol level >1.8 µg/dL (50 nmol/L) indicates Cushing's syndrome. 2
• Preferred for patients with renal failure (when UFC is unreliable) and for evaluating adrenal incidentalomas. 2
• Measure serum dexamethasone levels concurrently to reduce false-positive results from non-compliance or altered drug metabolism. 1, 2

Critical Test Selection Considerations

Avoid or interpret with extreme caution in these situations:

• Women taking oral estrogen: Estrogen increases corticosteroid-binding globulin (CBG), causing falsely elevated cortisol in DST; use UFC or LNSC instead. 2, 4
• Patients on CYP3A4 inducers (phenobarbital, carbamazepine, phenytoin, St. John's wort): These accelerate dexamethasone metabolism, causing false-negative DST results; use UFC or LNSC. 2
• Pregnancy: Use only UFC; DST is contraindicated. 2, 5
• Suspected cyclic Cushing's syndrome: Prefer UFC or multiple LNSC measurements over DST to capture intermittent hypercortisolism. 2, 5

Interpretation Algorithm

Concordant Positive Results (≥2 abnormal tests)

• Immediately refer to endocrinology and proceed with ACTH measurement to determine whether the cause is ACTH-dependent (pituitary or ectopic) or ACTH-independent (adrenal). 1, 2, 5
• Exclude pseudo-Cushing states (severe obesity, depression, alcohol use disorder, PCOS) before proceeding to imaging, as these conditions typically cause UFC elevations <3-fold the upper limit of normal. 2

Concordant Negative Results (≥2 normal tests)

• Cushing's syndrome is unlikely; no further work-up is needed. 3, 2
• Exception: If clinical suspicion remains high (e.g., progressive symptoms, adrenal incidentaloma with suggestive features), refer to endocrinology and consider re-evaluation in 6 months. 2, 5

Discordant Results (one positive, one negative)

• Obtain a third screening test or consider specialized tests such as midnight serum cortisol or dexamethasone-CRH test. 3, 2
• Refer to endocrinology for expert interpretation, especially if pretest probability is high. 3, 2

Common Pitfalls to Avoid

• Never rely on a single abnormal screening test before proceeding to imaging or invasive procedures. 2, 5
• Do not use random serum cortisol, plasma ACTH, or urinary 17-ketosteroids for initial screening. 2, 5
• Incomplete 24-hour urine collections invalidate UFC results; always verify collection adequacy with urinary creatinine. 4
• Pseudo-Cushing states (obesity, depression, alcohol use, PCOS) can mimic Cushing's syndrome biochemically but typically show UFC elevations <3-fold normal. 2
• Oral estrogen therapy, pregnancy, and chronic active hepatitis raise CBG and can falsely elevate total serum cortisol in DST. 4

Next Steps After Biochemical Confirmation

Once two concordant positive screening tests confirm hypercortisolism:

1. Measure plasma ACTH to differentiate ACTH-dependent (normal or high ACTH) from ACTH-independent (low ACTH) causes. 1
2. If ACTH is normal or elevated: Obtain pituitary MRI to look for an adenoma. 1
   • Lesions ≥10 mm are presumed Cushing's disease and do not require inferior petrosal sinus sampling (IPSS). 1
   • Lesions <6 mm require IPSS for confirmation. 1
   • Lesions 6–9 mm fall into a gray zone where expert opinion differs; consider CRH/DDAVP testing or IPSS. 1
3. If ACTH is low: Obtain adrenal CT or MRI to identify an adrenal source. 1